Herding has the advantage that it turns grass and plants inedible to humans into meat which has nearly everything they need. You get to keep moving around to graze lands and your food moves itself.

Nutritionally, is milk much different from meat? If they're similar, herding milk-producing livestock means you can feed low quality food that lies all over the ground to an animal and then get a near complete, meat-like diet without killing the animal.

Do we know if the first grain reserves were built to feed mainly humans or their livestock?

Pastoralism was a later development than agriculture.

Animal domestication is just as alengthy a process as plant domestication and the first domesticated animals were dogs, used for hunting other animals.

Early humans weren't nomadic, they tended to stick in one place. No animals to carry things for them, relying on intimate knowledge of sources of food and clean water, they found somewhere good and stayed there. The seaside was very popular, lots of shell fish and similar food sources.

In some parts of the world there were plants and animals that lent themselves to change through selection by humans, so they ended up with humans planting patches of fruitful crops, and eventually selecting for certain characteristics and bringing animals into the fold.

Some people then became nomads after they came into possession of animals capable of carrying burdens and carrying or dragging humans.